1. Field of the Invention
This invention is in the field of cooking of wood chips in a digester in the manufacture of pulp for papermaking and is specifically involved with a method of circulating liquors used in t:e process to bring the digester up to cooking temperature efficiently and rapidly.
2. Description of the Prior Art
In conventional cooking of wood chips in a digester, the temperature is raised to cooking temperature either by using indirect heating of the cooking liquor with a circulation system including a heat exchanger, or by direct heating which involves injecting steam directly into the digester. Both of these operations are time consuming, usually taking from 45 to 90 minutes. The indirect system is more efficient from an energy standpoint than the direct injection system.
In various cooking installations of the batch type, there is a significant variation in the usage of steam from steam generating plants depending upon which portion of the cycle is being carried out. This is an inefficient use of steam and significantly adds to the cost of the overall operation.
A prior art disclosure having some similarity with the subject matter of the present invention will be found in Rosenblad U.S. Pat. No. 2,882,148. In accordance with this disclosure, a digester is filled with a liquid of considerably lower temperature than the cooking temperature, the liquid being pumped into the digester so that air or other gas contained in the digester during the operation will be displaced or forced out of the digester by the liquid. This so-called "penetration liquid" can be a cooking liquor of full or less than full strength. The penetration liquid which is pumped in is then rapidly displaced through an outlet from the digester by means of the hot cooking liquor which is pumped into the digester at the cooking temperature. By throttling the outlet pressure in the digester, gases dissolved in the hot cooking liquor remain dissolved therein and thus reduce the evaporation of the hot cooking liquor.